In typical control valves, a valve cage may provide guidance for a valve plug as the valve plug moves from a closed position in which the valve plug sealingly engages a valve seat to an open position in which the valve plug is disposed away from the valve seat. When the valve is in the open position, fluid flows from a valve inlet, passes through a passage between the valve seat and the valve plug, passes through the valve cage, and exits through a valve outlet. In addition to guiding the valve plug, a valve cage can also be used for additional functions, such as noise reduction.
Referring to FIG. 1, a typical control valve 10 is shown. Control valve 10 generally includes a valve body 12 having an inlet 14 and an outlet 16 and a passageway 18 disposed between inlet 14 and outlet 16. A valve seat 24 is disposed in passageway 18 between inlet 14 and outlet 16 and a solid cage 22 is disposed within valve body 12 adjacent valve seat 24. A fluid control member, such as valve plug 26, is positioned within body 12 and is disposed within cage 22. Valve plug 26 interacts with the valve seat 24 to control fluid flow through the body 12, such that valve plug 26 sealingly engages valve seat 24 in a closed position and is spaced away from valve seat 24 in an open position. A stem 28 is connected to valve plug 26 at one end and to an actuator 30 at another end. Actuator 30 controls movement of valve plug 26 within cage 22. The cage 22 is positioned adjacent valve seat 24 and proximate valve plug 26 to provide guidance for valve plug 26.
In some gas applications, cage 22 has a plurality of passages 20 formed through a circumferential wall of cage 22, which are used is to reduce the noise produced as the gas passes through cage 22. Passages 20 are spaced specifically such that the jets of gas that are produced as the gas exits passages 20 do not converge and produce aerodynamic noise. Cages 22 used in these types of gas applications are typically used in a “flow up” orientation (e.g., the gas enters the center of cage 22 and passes from an inside surface to an outside surface of cage 22) and the spacing of passages 20 that is crucial to reduce the aerodynamic noise is on the outer surface of cage 22. The spacing of passages 20 on the inner surface of cage 22 is also important, as this spacing is used to keep sufficient space between passages 20 to not allow flow to pass through more passages 20 than desired for accurate flow characteristics throughout the travel of valve plug 26.
For solid cages 22 used in gas applications where the process conditions produce aerodynamic noise as the medium flows through control valve 11, drilled holes through the circumferential wall of cage 22 are typically used to form passages 20. However, drilled hole cages are very cumbersome, time consuming, and costly to produce. Some drilled hole cages may contain thousands of holes and the only real feasible way to produce passages 20 was to drill them with a ⅛ inch drill bit. Acceptance criteria exists that allows a percentage of drill bits to break and be left in the cage and this process requires the use of special drilling machines that have a high degree of accuracy.
In addition to the spacing of passages 20 on the outer surface of cage 22, aerodynamic noise can also be reduced by providing a tortured, or non-linear, flow path for passages 20 or to varying the cross-sectional diameter of passages 20 as they pass through the wall of cage 22. However, with a drilled holes through a solid cage 22, creating passages 20 having a non-linear flow path or having a variable cross-sectional area is not possible.
In addition to the noise issues that can be encountered in some gas applications, in some liquid applications, conditions can occur that will produce a condition where the liquid cavitates, which can cause severe damage to control valve 10. In order to reduce the cavitation that can occur to the point that it does not damage control valve 10 or to direct it to an area that is less susceptible to cavitation damage, passages that decrease in diameter in the direction of fluid flow can be used.
However, using drilled holes and conventional manufacturing techniques to create passages 20 in a solid cage 22 requires that the holes be step drilled from the outer surface of the cage, which limits these holes to having the larger diameter portion of passage 20 on the outer surface of cage 22 and the smaller diameter portion of passage 20 on the inner surface of cage 22, since the larger diameter portion has to be drilled from the outside of cage 22. This limits these types of cages 22 to applications using a “flow down” orientation (e.g., the fluid enters cage 22 from the outer surface and passes from the outside surface to the inside surface of cage 22) so that the pressure drops can be reduced as the flow goes through the control valve 10 and then downstream. The overriding reason this is done in this manner is the ability to drill the stepped holes from the outside of cage 22. As described above, drilling the number of holes required through the wall of cage 22 for this type of application is very cumbersome, time consuming, and costly to produce.